Radio apparatus for detecting aircraft



y 9, 1940- D. L. PLAISTOWE 2,207,267

RADIO APPARATUS FOR DETECTING AIRCRAFT Filed July 11, 1938 11 mnwmrrm 10 .9 a MODULATOR Jmc/mo/wzm RECEIVER une" FREQUENCY 16 (a OJC/LLAIO lFfCE/VER 19 525%?016 .18 6 7 1 Rf Z .s'Awroaru a an 1 gas/warm v RECEIVER 20 CJOURCE A b 6 b MIXER gnoentor Donald L. Plaistowe' Clttorneg Patented July 9, 1940 UNITED STATES RADIO APPARATUS FOR DETECTING AIRCRAFT Donald Leopold Plaistowe, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application July 11, 1938, Serial No. 218,656 In Great Britain August 13, 1937 6 Claims.

This invention relates to radio apparatus for detecting the presence of aircraft.

The invention is based upon the fact that an aircraft when flying in the field of an ultra short wave transmitter, is capable of reflecting waves therefrom and that reflected waves obtained at an ultra short wave receiver from a transmitter via reflection from an aircraft will arrive with a time interval (which will vary as the aircraft moves) with respect to the arrival of corresponding waves which have travelled direct from the transmitter to the receiver.

According to this invention a radio system for detecting the presence of aircraft comprises a reproducer apparatus of the television type adapted to produce atele'vision scan" or "raster in any manner well known from the television art; means for supplying said apparatus with periodic signals to enable it to produce the said raster; a very short wave radio transmitter; means for modulating the waves transmitted from said transmitter with a series of periodic signals occurring at a frequency correlated with the scanning line frequency in the raster; a plurality of very short wave radio receivers distributed over the area in which detection is to be effected; a transmission line system connecting said receivers with the point where the television reproducer apparatus is located; and means for applying as picture signals to said television reproducer apparatus signals derived from one or more of the receivers and sent over the transmission line system.

Preferably the signals transmitted from the transmitter consist of a series of pulses of the same frequency as the scan line frequency employed for the raster and preferably also means are provided whereby the signals derived from any one or more of the receivers may be applied to the television reproducer apparatus at will.

A preferred system in accordance with this invention will now be described with the aid of the accompanying drawing which shows the system in block diagram form. Referring to the drawing I is a cathode ray tube of the type now well known for television reception and having an electron gun with the usual control electrode-the control electrode is represented at 2-a fluorescent screen 3 at the end of the tube, and mutually perpendicular sets 4, 5, of ray deflecting means-for example pairs of plates as shown-one for deflecting the cathode ray in the line direction and the other for deflecting the cathode ray in the framing direction. A master alternating current source 6, for example a synchronised generator operating at (say) 50 cycles per second, is employed to drive any known form of time base circuit 1 (also operating at 50 cycles per second; say) which provides sawtooth oscillations which am: am (me nair (4) of plates to deflect the cathode ray in the framing or scanning direction. A line frequency time base circuit 8 of any suitable known type operating, for example at 10,000 periods per second, feeds line deflecting saw tooth waves to the line deflecting plates 5 of the cathode ray tube. This line time base circuit may, if desired, be driven through suitable frequency multipliers from the master source 6 in known way. Output from the line time base circuit 8 is fed to any suitable network 9 designed to provide an output consisting of short length substantially rectilinear synchronising pulses at the line frequency (10,000 periods per second in the present example) and this output is fed to a modulator in which modulates a very short wave radio transmitter ll preferably in such manner that the modulated impulses are of longer time length than the return time of the synchronising impulses. For example if the return time of line frequency signals is onehundred-thousandth part of a second, the modulation pulses actually transmitted may conveniently be made one-fifty-thousandth part of a second in length. The duration of each pulse, therefore, is small with respect to the spacing between successive pulses. The framing frequency signals are not transmitted. The transmitter ll feeds a transmitting aerial I2.

Distributed substantially uniformly throughout the area over which aircraft detection is to be effected is a plurality of ultra short wave receivers within the direct ray service area from the transmitter. Four such receivers are represented at l3, l4, l5, [6 in the figure though there may be any number of such receivers. Each of these receivers is connected by a cable or transmission line l1, l8, I9, 20, to a mixer stage 2| near the cathode ray tube and preferably switches 22, 23, 24, 25, are provided at the mixer stage 2| whereby the said mixer stage may be fed as may be desired with pulses picked up by one or any combination of the receivers. The output from the mixer stage is fed to the control electrode 2 of the cathode ray tube l to modulate the intensity of the cathode ray therein.

With this arrangement, assuming that there is no radio wave reflector (that is to say no aircraft) anywhere near any of the receivers and that all the switches are closed, there will be seen upon the fluorescent screen of the cathode ray tube a "raster upon which (assuming that the framing direction is the vertical direction) a vertical black bar will appear, this bar being due to the rectilinear pulses picked up by the receivers which are fed to the control electrode 2 of the tube I.

Since the radiated pulse is longer than the return time of the line frequency voltage, the impulse must be applied to the tube during a portion of the scanning cycle. If there are four receivers feeding into the mixer stage, each will produce quently, the differences in the lengths of the radio frequency paths produce only a small shift in the phase of the received pulse with respect to the synchronized line drawing voltage. If,

however, an aircraft flies near any one of the receivers, that receiver will pick up, not only the direct waves from the radio transmitter, but also indirect waves reflected from the aircraft and these indirect waves will not reach the receiver at the same time as the direct waves, and'this time difference is an appreciable portion of the period of the carrier frequency. Accordingly, the

carrier frequency component of the reflected signals reflected from the moving aircraft will suffer phase changes with respect to those directly received, which depend upon the length of the path of the reflected signals with respect to a wave length at the carrier frequency, and the bar image which will appear vertically across the raster will change from black to white and'back again as the length of the reflected path increases or decreases by one wave length at the carrier frequency as the aircraft travels on its course. The white image is due to interference between two simultaneously received impulses whose carrier frequency currents are for the moment in phase opposition, thus partially or completely cancelling the received impulse. The black image is due to the action of an impulse on the grid of the cathode ray tube, as pointed out above. Thus, if the switches connecting the receivers of the'mixer stage are all closed any aircraft anywhere near any of the receivers will immediately manifest its presence by bar images of changing intensity appearing on the screen of the cathode ray tube. By then opening theswitches and closing them one at a time in succession, it can be determined with a fair degree of accuracy which of the receiving stations the air= craft is nearest, by noting which receiver produces the bar having the greatest change, especially if, as will usually be thecase, the reception conditions for the detection area are known;

I claim as my invention:

1. A radio system for detecting the presence of a foreign body within a region to be protected which includes a cathode ray tube having a pair of ray deflecting means for deflecting said ray in angularly disposed directions; a control electrode for varying the intensity of said ray; means for applying a scanning voltage to one of said'deflecting means; means for applying a line deflecting voltage to the other of said deflecting means; the frequency of said line deflecting voltage being an exact multiple of said scanning voltage; means for deriving modulating pulses from said line drawing voltage; means for modulating an ultra high frequency carrier by said modulating pulses; means'for receiving said modulated carrier pulses directly and after reflection from said foreign body; and means for applying said received pulses to said control electrode. 7

2. A radio system for detecting the presence of a foreign body within a region to be protected which includes a cathode ray tube having a pair of ray deflecting means for deflecting said ray in angularly disposed directions; a control electrode forvarying the intensity of said ray; means for applying a scanning voltage to one of said deflecting means; means for applying a line deflecting voltage to the other of said deflecting means; the frequency of said line deflecting voltage being an exact multiple of said scanning voltage; means for deriving discrete modulating pulses from said line drawing voltage; the duration of said pulses being small with respect to the spacing between successive pulses; means for modulating an ultrahigh frequency carrier by said modulating pulses; means for receiving said modulated carrier pulses directly and after reflection from said foreign body; and means for applying said received pulses to said control electrode.

- 3. A radio system for detecting the presence of a, foreign body within a regionv to be'protected which includes an oscillograph having beam-producing means; means for deflecting said beam in a first direction in response to a first deflective voltage; means for deflecting said beam in a second direction in response to a second deflecting voltage; said first and second deflecting voltages having differentfrequencies and a fixed phasal relation; a screen interposed in the path ofv said beam for making visible the trace, .of said beam;

means for radiatingshort pulses of high -frequency radio energy; the period of said pulses being equal to'the period of one of said deflecting voltages; means for receiving said radio frequency pulses directly and after reflection from said foreign body; and mea-ns'for varying the intensity of said beam in accordance withthe amplitude of said received pulses.

4; A radio system for detecting the presence of a foreign body within a region to be protected which includes an oscillograph having beamproducing means; means for deflecting said beam in a first direction in response to a scanning voltage; means for deflecting said beam in a second direction in response to a line drawing voltage, the frequency of said line drawing voltage being an exact multiple of the frequency of said scanning. voltage; means interposed in the path'of said-beam for making visible the trace of said beams in a plane which includes said first and second directions; means-for radiating discrete pulses of high frequency radio energy, the duration of said pulses being short with respect to the period of said line drawing voltage; and the region for receivingv said pulses and means for applying the received pulses from any desired number of said plurality to said control electrode.

6. A device of the character described in claim 1, in which said means for receiving said pulses includes a plurality of .ultra high frequency receivers located at spaced points throughout said region, each of said receivers being connected to said control electrode by' a transmission line including a switch.

DONALD LEOPOLD PLAISTOWE. 

